TOPIC
about_Foreach
SHORT DESCRIPTION
Describes a language command you can use to traverse all the items in a
collection of items.
LONG DESCRIPTION
The Foreach statement (also known as a Foreach loop) is a language
construct for stepping through (iterating) a series of values in a
collection of items.
The simplest and most typical type of collection to traverse is an array.
Within a Foreach loop, it is common to run one or more commands against
each item in an array.
Syntax
The following shows the Foreach syntax:
foreach ($<item> in $<collection>){<statement list>}
The Foreach Statement Outside a Command Pipeline
The part of the Foreach statement enclosed in parenthesis represents a
Variable and a collection to iterate. Windows PowerShell creates the
Variable ($<item>) automatically when the Foreach loop runs. Prior to
each iteration through the loop, the Variable is set to a value in the
collection. The block following a Foreach statement {<statement list>}
contains a set of commands to execute against each item in a collection.
Examples
For example, the Foreach loop in the following example displays the
values in the $letterArray array.
$letterArray = “a”,”b”,”c”,”d”
foreach ($letter in $letterArray)
{
Write-Host $letter
}
In this example, the $letterArray array is created and initialized with
the string values “a”, “b”, “c”, and “d”. The first time the Foreach
statement runs, it sets the $letter Variable equal to the first item in
$letterArray (“a”). Then, it uses the Write-Host cmdlet to display the
letter a. The next time through the loop, $letter is set to “b”, and so
on. After the Foreach loop displays the letter d, Windows PowerShell
exits the loop.
The entire Foreach statement must appear on a single line to run it as a
command at the Windows PowerShell command prompt. The entire Foreach
statement does not have to appear on a single line if you place the
command in a .ps1 script file instead.
Foreach statements can also be used together with cmdlets that
return a collection of items. In the following example, the Foreach
statement steps through the list of items that is returned by the
Get-ChildItem cmdlet.
foreach ($file in Get-ChildItem)
{
Write-Host $file
}
You can refine the example by using an If statement to limit the results
that are returned. In the following example, the Foreach statement
performs the same looping operation as the previous example, but it adds
an If statement to limit the results to files that are greater than 100
kilobytes (KB):
foreach ($file in Get-ChildItem)
{
if ($file.length -gt 100k)
{
Write-Host $file
}
}
In this example, the Foreach loop uses a property of the $file Variable
to perform a comparison operation ($file.length -gt 100k). The $file
Variable contains all the properties in the object that is returned by
the Get-ChildItem cmdlet. Therefore, you can return more than just a
file name. In the next example, Windows PowerShell returns the length and
the last access time inside the statement list:
foreach ($file in Get-ChildItem)
{
if ($file.length -gt 100k)
{
Write-Host $file
Write-Host $file.length
Write-Host $file.lastaccesstime
}
}
In this example, you are not limited to running a single command in a
statement list.
You can also use a Variable outside a Foreach loop and increment the
Variable inside the loop. The following example counts files over 100 KB
in size:
$i = 0
foreach ($file in Get-ChildItem)
{
if ($file.length -gt 100k)
{
Write-Host $file “file size:” ($file.length /
1024).ToString(“F0”) KB
$i = $i + 1
}
}
if ($i -ne 0)
{
Write-Host
Write-Host $i ” file(s) over 100 KB in the current
directory.”}
else
{
Write-Host “No files greater than 100 KB in the current
directory.”
}
In the preceding example, the $i Variable is set to 0 outside the loop,
and the Variable is incremented inside the loop for each file that is
found that is larger than 100 KB. When the loop exits, an If statement
evaluates the value of $i to display a count of all the files over
100 KB. Or, it displays a message stating that no files over 100 KB were
found.
The previous example also demonstrates how to format the file length
results:
($file.length / 1024).ToString(“F0”)
The value is divided by 1,024 to show the results in kilobytes rather
than bytes, and the resulting value is then formatted using the
fixed-point format specifier to remove any decimal values from the
result. The 0 makes the format specifier show no decimal places.
The Foreach Statement Inside a Command Pipeline
When Foreach appears in a command pipeline, Windows PowerShell uses the
foreach Alias, which calls the ForEach-Object command. When you use
the foreach Alias in a command pipeline, you do not include
the ($<item> in $<collection>) syntax as you do with the Foreach
statement. This is because the prior command in the pipeline provides
this information. The syntax of the foreach Alias when used in a command
pipeline is as follows:
<command> | foreach {<command_block>}
For example, the Foreach loop in the following command pipeline displays
any processes whose working set (memory usage) is greater
than 20 megabytes (MB). Windows PowerShell pipes the output from the
Get-Process command to the foreach Alias. Inside the foreach Alias
command block, the $_.WS Variable contains the value of the WS (working
set) property passed to it by the Get-Process cmdlet. (The $_ portion
of the declaration is a Windows Script Host [WSH] automatic Variable,
and the WS portion is a property). The If statement uses a conditional
statement to determine whether the working set is greater than 20 MB
(20,000,000 bytes). If so, the name of the process that is stored in
the $_.name Variable and the working-set size in megabytes are displayed.
If no process working set is over 20 MB, nothing is displayed.
Write-Host “Processes with working-sets greater than 20 MB”
Get-Process | foreach {
if ($_.WS -gt 20m)
{
Write-Host $_.name “: ”
($_.WS/1m).ToString(“F0”) MB -Separator “”
}
}
The foreach Alias also supports beginning command blocks, middle command
blocks, and end command blocks. The beginning and end command blocks run
once, and the middle command block runs every time the Foreach loop steps
through a collection or array.
The syntax of the foreach Alias when used in a command pipeline with a
beginning, middle, and ending set of command blocks is as follows:
<command> | foreach {<beginning command_block>}{<middle
command_block>}{<ending command_block>}
The following example demonstrates the use of the beginning, middle, and
end command blocks.
Get-ChildItem | foreach {
$fileCount = $directoryCount = 0}{
if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}{
“$directoryCount directories and $fileCount files”}
The beginning block creates and initializes two Variables to 0:
{$fileCount = $directoryCount = 0}
The middle block evaluates whether each item returned by Get-ChildItem
is a directory or a file:
{if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}
If the item that is returned is a directory, the $directoryCount
Variable is incremented by 1. If the item is not a directory,
the $fileCount Variable is incremented by 1. The ending block runs after
the middle block completes its looping operation and then returns the
results of the operation:
{“$directoryCount directories and $fileCount files”}
By using the beginning, middle, and ending command block structure and
the pipeline operator, you can rewrite the earlier example to find any
files that are greater than 100 KB, as follows:
Get-ChildItem | foreach{
$i = 0}{
if ($_.length -gt 100k)
{
Write-Host $_.name “file size:” ($_.length /
1024).ToString(“F0”) KB
$i++
}
}{
if ($i -ne 0)
{
Write-Host
Write-Host “$i file(s) over 100 KB in the current
directory.”
}
else
{
Write-Host “No files greater than 100 KB in the current
directory.”}
}
SEE ALSO
about_Automatic_Variables
about_If
ForEach-Object